Luminescent rare-earth doped alkaline-earth fluorides have long been known, and have been employed for numerous purposes such as scintillation detectors and laser materials. CaF2 doped with such rare-earth species as Eu+3, Er+3, Tb+3 are well-known compositions. It is well-known that a rare-earth doped alkaline earth fluoride will exhibit luminescence when exposed to ultraviolet light.
Each rare-earth element when incorporated into an alkaline earth host lattice such as CaF2 exhibits a characteristic excitation spectrum; see, for example, FIG. 1 (101), and a characteristic emission or luminescence spectrum that depends upon the excitation wavelength employed; see, for example, FIG. 1 (102). The excitation spectrum is determined by monitoring the luminescence intensity at one wavelength while the specimen is illuminated over a range of wavelengths. The luminescence spectrum is determined by illuminating the specimen at a single wavelength corresponding to a peak in the excitation spectrum and determining the luminescence spectrum by scanning a detector over a range of wavelengths.
As shown in the figures, each such spectrum consists of a plurality of peaks at different wavelengths of light. The wavelengths at which the peaks occur are characteristic of each rare-earth element. No two rare-earth elements exhibit the same excitation or emission spectra; that is, the peaks in their spectra do not in general arise at the same wavelengths. To obtain luminescence, the rare-earth element must be excited by a light source that emits light at a wavelength corresponding to the location of one of the peaks in the excitation spectrum thereof. In general, the peaks in any one spectrum of rare-earth elements differ from one another in height or intensity, these differences in intensity being characteristic of the rare-earth element under particular conditions of measurement. These and related matters are all well-documented in the art. See for example, Martin et al., Atomic Energy Levels—the Rare-Earth Elements, U.S. Department of Commerce, National Bureau of Standards (1978).
Copending application 60/687,646 discloses a room temperature aqueous solution based method for preparing rare-earth doped alkaline earth fluoride nanoparticles.
Sarma et al., Solid State Ionics 42, 227 (1990) discloses solid state solutions of CaF2 and SrF2. No mention is made of rare-earth doping.
Federov et al., Doklady Akademii Nauk. 369(2):217-219, 1999, discloses solid solutions consisting of a series of 10 mm diameter and 50 mm long single crystals of (Ca1−ySry)1−xNdxF2+x grown by the Bridgman-Stockbarger method by crystallization from the melt.
Considerable effort in the art is being directed towards developing luminescent compositions for use as identifying marks on commercial goods, including packages, manufactured articles, and even money. One idea is to place an identifying mark on a manufactured article which will attest to its authenticity in the face of rampant piracy on a global scale. The mark is ideally invisible until inquiry is made using a particular wavelength of light which then stimulates luminescence with a characteristic spectrum.
A simple luminescent security mark may itself be easy to counterfeit. The present invention provides a family of novel rare-earth-doped alkaline earth fluorides, and a process for preparing them, that are characterized by unique luminescence peak intensity ratios, making it extraordinarily difficult to counterfeit security marks comprising these compositions.